This invention relates in general to sewing machines and, in particular, to a new and useful device for initiating the movement of the thread catcher of a thread cutting device whose actuating linkage comprises a sensing lever adapted to be coupled with a control disc drivable synchronously with the looper shaft of the sewing machine.
A thread cutting device for sewing machines is known from German Pat. No. 1,125,742, wherein the movement of the thread catcher is initiated by means of a hand lever which may be actuated only during standstill of the machine, via a shift linkage. A locking lever cooperating with a cam plate is pivoted in a certain position of the cam plate to release the pawl of an entrainer disc. The pawl is secured on an intermediate shaft connected with the looper drive shift, so that the pawl comes into operative position, in which it comes into engagement with a driver pin secured on a gear loosely mounted on the intermediate shaft and connected with the looper drive shaft via a gear pair. Via the pin and the pawl, as the rotation of the machine continues, the entrainer disc is driven by the intermediate shaft, from which via a crank, a coupling and a disc loosely mounted on the looper drive shaft, the movement required for thread cutting is imparted to the thread catcher. A cam plate provides for the control of the locking lever to bring the pawl into its inoperative position outside the movement path of the pawl at the end of the cutting process.
Since this cutting device can be operated for thread cutting only at standstill of the machine, it has been proposed by German Pat. No. 1,159,247, to provide a shifting shaft parallel to the looper drive shaft, consisting of two sections connected by a compensating clutch, with one section being non-displaceable axially, while the other section is axially movable. The axially non-displaceable section is connected with the thread cutting device, while the other section can be brought into operative connection with a control disc secured on the looper shaft. A compression spring is disposed on this section which takes support at one end against a stop secured on it and at the other end against an axially displaceable bushing disposed on this section for axial displacement and which applies against a hand- or foot-operated shifting fork. An abutment ring is arranged on the axially movable section, the end face of which cooperates with a locking lever in a drive connection with the looper drive shaft via a friction clutch. The locking lever applies against the axially movable section next to the end face of the abutment ring due to the torque acting on it via the friction clutch when the machine is running. If the bushing is axially displaced by the shifting fork and the compression spring is thus tensioned, to initiate the thread cutting process while the machine is running, the locking device will prevent a displacement of the axially movable section. The switch-on pulse, is therefore stored.
After the machine has been stopped, the looper drive shaft rotates back a small amount counter to its operative direction of rotation as a result of the return energy stored in the elastic drive belt of the machine. The locking lever is lifted and releases the abutment ring and hence the switch-on pulse. The axially movable shifting shaft section is suddenly displaced by the slackening compression spring so that a lag pin, fastened to a crank of the section, penetrates into the cam groove of the control disc. As rotation of the machine continues, according to the form of the cam groove, a pivotal movement is transmitted by the crank to the axially movable shifting shaft section and conveyed on via the compensating clutch to the axially non-displaceable section which, via a crank and a coupling, imparts to the thread catcher the movement required for seizing and severing the thread. In this device, the shift fork must be held in its operative position for the duration of the cutting process since, if inadvertently let go, the cutting process would be interrupted prematurely. Thus, some uncertainty exists with respect to the completion of the cutting process.
It can be seen that both devices require considerable cost of engineering for the initiation and control of the thread cutting device. Also unsatisfactory is the fact that the coupling of the gear parts of the thread catcher with the control or entrainer disc causes a percussion noise and the parts are under severe stress by the sudden impact.
The simplification of such devices has been attempted by using an electromagnet as a drive means for the thread catcher (German Pat. No. 1,485,265). Here, the operative movement of the thread catcher proceeds in a sudden stroke of the magnet all the way counter to the action of a return spring, and thus occurs during standstill of the machine in a certain position of the needle in which the threads occupy the correct position for being seized by the thread catcher. However, it is relatively difficult to provide for the exact control of this position, and there is great danger that the position of the threads will vary due to their inherent elasticity after stoppage of the machine, so that the thread catcher does not seize the threads or does not correctly engage the threads.